Lithographic printing plate and method of making



Nov. 2, 1954 I w. a. MULLEN LITHOGRAPHIC PRINTING PLATE AND METHOD OF MAKING Filed March 9, 1948 [220670507 "3 WJZ'QWZ @Mullew United States Patent LITHOGRAPHIC PRlNTlNG PLATE AND METHOD OF MAKING William G. Mullen, Winchester, Mass, assignor, by mesne assignments, to A. B. Dick Company, Cook County, 111., a corporation of Illinois Application March 9, 1948, Serial No. 13,803

4 Claims. (Cl. 101149.2)

This invention relates to printing plates of the type having a surface capable of receiving and retaining a lithographic ink or the like. 1

Such printing plates may be made from various materials capable of providing a surface having lithographic properties and apart from plates having a grained metallic surface, such as zinc, copper, luminum, etc., these plates usually comprise a support, such as a sheet of treated paper, having on its surface a coating consisting essentially of a hydrophilic colloid, such as a water-soluble protein, polyvinyl alcohol or the like material, adapted to be hardened or insolubilized by reaction with an aldehyde, a chromium compound, alum, a formate or the like compound, which chemically reacts with the colloid to render the surface of the coating more or lessreceptive to lithographic inks. Illustrative of such prior art printing plates and procedures are the patents to Shepherd, 2,154,219, granted April 11, 1939, and Simons, 2,132,443 and 2,156,100, granted October 11, 1938, and April 25, 1939, respectively.

An outstanding problem in the manufacture of such plates is the difficulty of accurately controlling the hardening action of the insolubilizing compound, particularly where, as is usual, the coating contains an alkali pigment, such as satin white, casein or the like material. If the hardening agent is applied to the surface of the coated sheet or plate, it is practically impossible to control its penetration and absorption and hence its effectiveness.

If an excess of hardening agent is used, or if the treatment is prolonged, or carried out at too high a temperature, the coating is over-hardened, with the result that it is over-receptive to lithographic ink and insufficiently water-receptive to permit satisfactory printing. On the other hand, if the coating is under-treated, it is highly water-sorptive and hence exhibits a poor grease receptivity, which also renders the plate unsatisfactory.

Although it is possible by the exercise of careful control of all manufacturing steps to produce a plate having a coating hardened or insolubilized to the desired degree, it is found that such plates, if used within a relatively short period of time after manufacture, are generally satisfactory, but that if the period between manufacture and use exceeds a few months, a progressive hardening action of the coating appears to take place and consequently such plates become unsatisfactory due to poor water-receptivity. As far as I am aware, no practical method for producing a stabilized coating on such plates has heretofore been found.

The principal objects of the present invention are to overcome the before-mentioned difficulties and to provide a printing plate which has a stabilized colloidal coating possessing lithographic properties, which can be used in both slow and fast presses, and which is capable of producing a large number of impressions comparing favorably with those produced by metallic plates, and to'provide an efficient and reliable method of making such plates.

Further objects will be apparent from a consideration of the following disclosure.

I have observed that when the colloidal coating of a printing plate is treated in accordance with the usual practices to harden or insolubiilze the colloid, at least the outer surface of the resulting coating is rendered relatively impermeable to water, depending upon the degree of insolubilization produced, even though the coating as a whole may be far from water-repellent. Hence, the nonprinting areas of the coating and underlying sub- Patented Nov. 2, 1954.

surface stratum cannot reliably serve as a reservoir to hold the necessary amount of moisture to insure good printing. 1

I have discovered that the colloidal coating of such plates may be treated, in the manner hereinafter described, so as to confine the hardening action substantially to the surface and immediate subsurface stratum of the coating, thereby to produce an insolubilized sur= face layer or membrane which is relatively permeable to water and yet grease receptive, and an under layer which is relatively insolubilized, but yet highly water-sorptive. Hence, the under surface, being water-sorptive, acts as a reservoir for receiving and holding the moisture necessary to maintain the nonprinting areas in the moist condition necessary to insure good printing.

The method by which the foregoing is accomplished consists essentially in incorporating in the colloidal coating a light filtering substance such as a. relatively opaque filler uniformly dispersed throughout the coating and/or a water-soluble yellow dye, together with a water-soluble photosensitive material, such as ammonium, potassium or sodium bichromate, after which the photosensitive coating is subejcted to actinic light for a period sufficient to produce the desired results. The opaque filler and/0r yellow dye, as the case may be, acts as a filter effective to confine the penetration of at least the major portion of the light rays to the surface and the immediate subsurface stratum of the coating. Since the hardening of a photosensitive coating depends upon the penetration of the light rays, it will be seen that by confining the penetration to the surface and immediate subsurface stratum of the coating, a differential hardening action is necessarily produced, and by controlling the intensity of light and/or period of exposure, any desired degree of hardness within practical limits can be attained. Hence, a colloidal surface possessing the desired lithographic properties may readily be produced without efi'ecting a corresponding hardening of the inner layer or base stratum of the coating. After having subjected the colloidal coating to the photohardening treatment, the photosensitizing material is washed out by subjecting the treated plate to a suitable aqueous treatment, such as washing or soaking in water, thereby avoiding the danger of progressive hardening of the coating which might otherwise take place.

the relatively opaque pigments suitable for use as light filter comprise satin while, clay, titanium dioxide, barium sulfate, zinc sulfide, and the like light-colored conventional pigments, and mixtures thereof. Although colored pigments may be used, as a practical matter, their use is not recommended since they necessarily impart a coloration to the coating which is not always desirable. The term yellow dye, as herein used, connotes only those dyes which are in fact water-soluble and compatible with both the colloid and an aqueous bichromate solution, and which are capable of absorbing light of wavelengths from about 40o millimicrons or less to about 480 millimicrons. For example, a dye having an absorption characteristic approximately corresponding to a vvratten No. 5 filter is recommended as a generally satisfactory type.

Virtually all water-soluble yellow dyes having the above-specified absorption characteristics are operative so long as they are compatible with the hydrophilic colloid and bichromate solution, and as recognized technical classification, such as set forth in the Colour Index, Matthews, etc., are of little help in determining compatibility, it is advisable, if not generally necessary to test out the compatibility of a selected dye before use. To this end a 3% dye solution may be added to an aqueous dispersion of the hydrophilic colloid containing about the same amount of ammonium bichromate as the added dye, and if no precipitation or appreciable loss of color takes place, the dye may be considered compatible. Acid dyes constitute the preferred type, although direct dyes are satisfactory. Basic dyes are not generally so reliable because of the length of time usually required for the dye to be absorbed by the colloidal coating.

In accordance with the present invention a suitable support is coated with a photosensitizable hydrophilic colloid in accordance with conventional procedures. The support may consist of a metal plate or sheet, such as copper,

aluminum, zinc, steel, etc., a sheet .of suitable .plastic material, such as a water-resistant cellulose derivative, a vinyl resin sheet, etc., or a sheet of fibrous material, such as heavy paper, fiber board, or the like, previously impregnated or otherwise treated with a water-resistant composition so that it will not undergo appreciable .dimensional changes when subjected to an aqueous treatment.

The photosensitizable hydrophilic colloid may comprise such conventional materials as gelatin, albumen, casein or metal caseinates, soy bean, or the like protein, or a nonproteinaceous material, such as polyvinyl alcohol, carboxy methyl cellulose, or a mixture of any two or more of the above specified colloids. The entire hydrophilic coating is preferably rendered at least partially insolubilized by incorporating thereina small quantity of a relatively weak insolubilizing agentsuch as an aldehyde derivative, e. -g., hexamethylenetetramine, an hydroxide of a polyvalent metal, such as barium, calcium, magnesium and zinc, in an amount between 2% and 20%, the preferred working limits being between 5 and based on the dry weight of hydrophilic colloid. In addition, the incorporation of pigments of the above-mentioned type is advisable since such pigments materially assist in effecting a uniform partial insolubilization of the hydrophilic coating, and in some cases a pigment such as satin white may of itself be relied on to effect a partial insolubilization of the hydrophilic colloid. Although it is permissible to employ smaller amounts of other insolubilizing agents which are more active, e. g., alum, metal formates, formalin, etc., the activity of such relatively strong agents is usually too great to permit reliable and satisfactory control in the usual production methods and hence the use of such agents is not generally recommended.

though partial insolubilization may be effected by the action of light on the photosensitized material, the penetration of light into the under stratum is not always sufiicient' to effect the desired degree of insolubilization. In other words, the rate of hardening of the outer surface and subsurface stratum is considerably greater than the rate of insolubilization of the under layer or base stratum of the coating. Hence,.it is usually desirable, if not necessary, to incorporate in the coating composition a relatively weak insolubilizing agent and/ or a pigment in order to insure adequate insolubilization and maintain a more accurate and reliable control, but in any event the degree of insolubilization of the base stratum should be merely suflicient to prevent disintegration when subjected to aqueous treatment, but without material loss of watersorptivity.

In some cases the colloidal coating may be applied directly to the support, depending upon the character of its surface, but it is usually advisable, if not'necessary, to employ a prime coat so as to insure a satisfactory adhesion of the colloidal coating to the support. The coating operation may be carried out on any conventional type of coating machine and the weight of coating may vary from approximately 100 ounces to approximately 250 ounces or more per ream (24 x 36-480). Although the coating may be applied as a single coat, more satis factory results are generally obtained by applying two or more successive coats.

Where, as is preferred, the coating is to be partially insolubilized principally by the incorporation of a relatively weak insolubilizing agent, and where an opaque filler, with or without a yellow .dye, is to be relied on as the light-filtering substance, the opaque filler is thoroughly dispersed in the coating composition in an amount which may vary from less than 50% to more than 500% of the dry weight of the hydrophilic colloid, depending upon the character and opacity of the filler.

The drying of the coating composition, even when carried out at a relatively low temperature, is usually efiective to cause the insolubilizing agent to react with the colloid to an extent surcient to effect at least a partial insolubilization of the coating, without materially reducing its water sorptivity, and when employed in proper amounts the insolubilizing agent will be substantially depleted so that there is little dangerof over-insolubilizing the coating. If by the time the coating has been dried it has not been sufiiciently insolubilized, the subsequent imbibition, drying and light exposure will to some extent supplement the insolubilizing of the base stratum of the coating, and hence care must be exercised to avoid the use of excessive insolubilizing agent and drying temperature. When properly \coated and dried, the resulting coating is an insolubilized, water-sorptive, resilient layer firmly adherent .to .the support .and will not disintegrate when subjected to aqueous treatment.

After having thus prepared a coated support or sheet, the coating may then be rendered photosensitive by imbibition of a suitable composition comprising aqueous ammonium bichromate, or an alkali metal bichromate, in .an amount which may vary from approximately 0.5% to l0% of the dry weight of hydrophilic colloid, the preferred working range being between 2% and 5%. If desired, a yellow dye may be incorporated in the sensitizing solution either to supplement the action of the opaque filler, or to take the place of such filler, and when used the dye may be present in an amount which may vary from 0.1% to 10%, based on the dry weight of hydrophilic colloid, the preferred working limits being between 0.5% to 5%, depending upon the strength of the selected dye and its compatibility with the coating composition. It is usually advisable, though not necessary, to acidity .the bichromate=dye solution and to this end va small amount of aluminum formate maybe incorporated to produce a pH of about 4 to 5.

The period of imbibition may vary from approximately one second to sixty seconds .or more, a period .of between two seconds and thirty seconds being recommended. However, there appears to be nothing critical with respect to the period of imbibition and in virtually all cases .imbibition of sufficient bichromate takes place within a few seconds. After having sensitized the coating it may then be dried preferably at a temperature below F. in order to avoid heat-hardening. The dried, sensitized coating is next subjected to the action of a strong light as a carbon .arc, Cooper-Hewitt, or other light emitting a relatively high. percentage of blue light. The period of exposure may vary from a few seconds to fifteen minutes or more, depending upon the intensity of the light, amount of bichromate imbibed by the coating, and the particular hydrophilic colloid. For example, when the coating has imbibed a 1% aqueous ammonium bichromate solution, an exposure under :a 45 ampere, double carbon arc lamp at a distance of three .feet for .a period five to eight minutes, is entirely satisfactory, :but when a 5% bichromate solution has been used, an exposure of 45 to 60 seconds is satisfactory. The .optimum period of exposure for a specific coating composition having incorporated therein a predetermined amount of hichromate must be determined empirically. In any case the exposure effects a hardening of the surface and .immediate subsurface stratum of the coating, thus producing a water-permeable membrane having good lithographic properties, which is integral with the insolubilized, water-sorptive base stratum.

After having exposed the coating it is then washed with water for a period of about /2 to 5 minutes which is usually sufiicient to remove all unreacted bichromate and also any unreacted watersoluble insolubilizingagents, together with a major portion of the dye, if used. As a general rule one minute has been found sufficient to remove practically all the unreacted bichromate from a coating having a weight or" the order of 10 to 1.6 ounces per ream, and although all the dye may not be removed in this period, it is found that the presence of residual dye has no deleterious effect. The removal of unreacted bichromate together with any unreacted water-soluble insolubilizing agents results instabilizing the coating against a progressive hardening action which wouldotherwise take place, particularly if the printing plate were subjected .to any form of actinic light or heat, with the result that the printing plate would gradually lose its water-receptivity and ultimately be rendered useless. After the wash ing treatment the coating may be dried .in any suitable manner preparatory for use in direct image printing.

Although, as above indicated, it is preferable first to apply the coating composition to the support and subsequently render the coating photosensitive by irnbibition. if desired, all ingredients, including the bichromate, may be incorporated in the coating composition immediately before application to the support, and the coated support dried under conditions permitting accurate control of light, heat and other "factors influencing chemical action between the bichromate and colloid. In such cases the coating composition must be protected prior to and during its application against excessive light and heat to avoid promoting chemical action between .the bichromate and colloid. Moreover, the ,period of exposure of the coated sheet must "be adjusted and accurately controlled to avoid overhardening of the surface, and in any event the exposed sheet must be immediately washed to remove all unreacted insolubilizing agent and bichromate, thereby stabilizing the coating against progressive hardening. Since this alternative procedure requires more accurate control over all steps, its use is recommended only when suitable equipment is available.

Referring to the accompanying drawings which illustrate what is now considered a preferred embodiment of the invention:

Fig. l is a perspective view of a colloidal printing plate produced in accordance with the present invention;

Fig. 2 is a sectional perspective-on an exaggerated scale of the plate before treatment;

Fig. 3 is a schematic illustration of the successive steps employed in preparing printing plates in accordance with what is now considered a preferred procedure;

Fig. 4 is a view similar to Fig. 2 but showing the plate after treatment; and

Fig. 5 is a view similar to Fig. 2 showing the plate prepared for use.

In preparing a direct image printing plate in accordance with what is now considered a preferred procedure, a water-resistant fibrous sheet 1 (Figs. 1 and 2), prepared from a long-fiber bleached sulfate paper having a ream weight of about 1004i (24 x 36-480) is impregnated with an oil varnish synthetic resin or the like waterresistant material. The obverse face of the impregnated sheet is given a sealing coat with a pigmented resinous composition of the same nature as or compatible with the impregnating composition. The back or reverse face of the sheet is also given a sealing or protective coat to prevent exudation of the impregnation and to this end a casein-ester gum composition, a suitable lacquer or the like is applied, such coating preferably being pigmented or otherwise colored so as to identify the reverse face of the sheet. The obverse face of the sheet l is then prime coated, as indicated at 2 (Figs. 1 and 2), with a caseinresin dispersion to provide a surface receptive to the top coating 3. The impregnation, sealing and prime coatings may be applied in accordance with any of the conventional procedures.

The top coating may be applied in conventional manner either as a single coat, or as several individual coats, but preferably as a two-coat procedure, the first coat consisting of zinc caseinate, satin white, clay composition in the proportion of 1 part satin white to 1 part clay to 1 part caseinate (dry basis), and the weight of the coat is preferably of the order of 6 to 7 pounds per ream. The second coat is applied in the same manner and with the same composition, but is somewhat lighter, running to about 5 to 6 pounds per ream. The resulting sheet comprises a water-resistant base 1 having a prime coat 2 to which a pigmented casein top coat 3 is firmly adherent.

The coated sheet is now ready for treatment in accordance with the present invention and to this end the sheet S from a stock roll 10 (Fig. 3) is drawn through a trough 11 containing an aqueous solution of about 5% ammonium bichromate, 2% Tartrazine dye (or the equivalent yellow dye) and sufficient aluminum formate to produce a pH of 4 to 5. The length of the trough and rate of travel of the sheet through the bichromate-dye solution is such as to provide a period of imbibition of about one minute, and after passing from the trough 11, the sheet is then conducted through a dryer 12 where it is dried at a temperature of about 100 F.

The dried sheet is then conducted under a battery of carbon are lights 13 at a rate of travel which provides a period of exposure suflicient to effect hardening of the exposed surface of the top coat 3 which, as above explained, is governed by the number and size of lights and their distance from the surface of the sheet S, it being understood that these factors are correlated with the travel of the sheet so as to secure as close to optimum conditions as possible.

The exposed sheet is then conducted through one or more troughs 14 of water, the number and/ or length of which is such that the sheet is subjected to a washing period of about one to two minutes which is usually sufficient to wash out unreacted bichrornate.

After washing, the exposed sheet is then passed through a dryer 15 and the dried sheet is then wound up on a reel 16. The treated sheet thus produced (Fig. 4) comprises a water-resistant base 1 havin a coating 3 comprising a surface and immediate subsurface stratum which provides an insolubilized, water-permeable membrane 3* having lithographic properties and a base layer or stratum 3 which is relatively insoluble, but Water-sorptive.

The treated sheet may be cut into printing plates in accordance with conventional practice and when thus prepared, the individual printing plates are ready for use in direct image printing. As shown by way of example in Fig. 5, a character M to be reproduced is drawn or typed on the surface of the treated coating, using a lithographic ink. The plate is then subjected to an aqueous priming solution which penetrates the membrane 3 and is absorbed by the under layer or base stratum 3 which undergoes appreciable swelling, as indicated by a comparison of Figs. 4 and 5, thus providing, in effect, a reservoir for holding suflicient water to insure good printing. The photohardened membrane 3 provides a relatively tough printing surface to which lithographic ink is firmly adherent, but is sufiiciently water-permeable to permit water from the under layer 3 to migrate therethrough so as to maintain the nonprinting areas sufficiently moist to become grease-repellent.

Substantially the same procedure as above described may be employed in preparing various types of printing plates from other colloidal compositions. For example, an improved printing plate for high speed offset printing may be prepared from a coating comprising gelatin, insolubilized with from about 1% to about 1.5% alum (A12(SO4)3), and treated as above described.

1 A printing plate produced in accordance with the present invention not only has a much longer life than a similar plate made in accordance with prior practices, but furthermore is capable of producing prints of superior quality, particularly from originals having typed characters, figures or the like made from drawing ink, etc., as well as providing an excellent transfer plate. Moreover, my improved plate facilitates erasures since its hardened surface allows vigorous rubbing without danger of appreciable injury thereto.

While I have shown and described different embodiments of the invention, it is to be understood that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

I claim:

1. The method of making a direct image lithographic plate comprising the steps of coating a support with a hydrophilic colloid composition containing an insolubilizing agent in an amount sufficient partially to insolubilize and harden the colloid coating to a water-soluble, water sorptive stage and an opaquing substance to limit penetration of light therethrough, incorporating a light sensitive colloid insolubilizing substance in the colloid coating, exposing the entire surface of the applied coating uniformly to light for a time suificient to react the surface portion of the coating to an advanced stage of insolubilization but insufiicient to convert the hydrophilic surface of the coating to a water repellent, ink receptive stage, and then washing the colloid coating to eliminate unreacted light sensitive colloid hardening substance for stabilization of the coating.

2. The method of making a direct image lithographic plate comprising the steps of coating a support with a hydrophilic colloid composition containing an insolubilizing agent in an amount sufiicient partially to insolubilize and harden the colloid coating and a pigment opaque to light, reacting the coating to effect partial insolubilization and hardening of the coating throughout to a water sorptive water insoluble stage, incorporating a light sensitive colloid insolubilizing substance in the coating, exposing the entire top side of the coating to light for a time suflicient to react the surface portion of the coating to an advanced stage of insolubilization but insufficient to convert the hydrophilic surface of the coating to a water repellent, ink receptive stage, washing the coating to remove unreacted light sensitive colloid hardening substance for stabilization of the coating and. imaging the surface with an imaging material to form the ink receptive, water repellent image directly on the plate.

3. The method of making a direct image lithographic plate compjrising the steps of coating a support with a hydrophilic colloid composition containing an insolubilizing agent in an amount sufficient partially to harden and insolubilize the coating throughout and a water soluble yellow dye, reacting the coating to effect partial insolubilization and hardening thereof, incorporating a light sensitive colloid insolubilizing substance in the colloid coating, exposing the. entire top side of the doating to light for a time suflicient to react the surface portion of the coating to an advanced stage of insolubilization but insuificient to convert the hydrophilic surface of the coating to a water repellent, ink receptive stage, rinsing the coating to remove unreacted light sensitive colloid hardening substance for stabilization of the coating and imaging the surface with a greasy substance to form the ink receptive, water repellent image directly thereon.

4. A direct image lithographic. plate comprising a support, a continuous coating on the support formed of a hydrophilic colloid in which the colloid is reacted throughout to a partially hardened and insolubilized stage: by reaction with a chemical insolubilizing agent and in which the surface portion is reacted to a further stage of insolubilization by an insolubilizing agent reactive to exposure. to actinic light for a time sufficient to insolubilize without conversion of the surface portion to an ink re.- ceptive, water repellent stage, and an ink receptive, water repellent imaging material anchored to form the image on the insolubilized hydrophilic surface portion of the.- colloid coating.

References Cited in the file of this patent Number UNITED STATES PATENTS 

